Imaging systems, such as complementary metal oxide semiconductor (CMOS) imagers, are used in various imaging devices such as image sensors used in digital cameras, camcorders and other electronic products. A typical CMOS imager includes an array or matrix of pixel cells. Each pixel cell includes a photoconversion device for generating and accumulating a charge responsive to light shining on the photoconversion device. The light shining upon the pixel cell corresponds to an element of the overall picture, and the resulting electrical signal corresponds to the signal generating pixel's portion of the image. A pixel readout circuit is coupled to the photoconversion device, and provides a pixel readout of the photoconversion device.
A source follower readout circuit is a common configuration for a pixel readout circuit. The source follower readout circuit includes a source follower transistor having its gate (G) coupled to a photoconversion device, and its source (S) coupled to a row selector. The row selector provides selective activation of the source follower transistor. A bias transistor acts as a current source to pull current through the source follower transistor once the row selector switch is closed. An output node of the circuit is coupled to the source (S) of the source follower transistor, and provides an output signal representing the pixel readout.
Conventional source follower circuits have a variety of problems. First, there are sources of noise in the source follower circuits that are injected into the output signal. The bias source is one potential source of noise in a source follower circuit. Source follower circuits are unable to provide common-mode rejection to cancel this bias noise and other noise from the output signal. This results in a less than optimal pixel readout from the pixel readout circuit.
Additionally, there is a gain degradation in source follower circuits, which leads to a significant loss of the pixel readout. As imaging systems become smaller and more densely populated with components, the transistors in these imaging systems also become smaller. As a result, the output signals provided by these transistors also become smaller in magnitude and slight variations in voltages can cause significant signal losses, which can potentially affect the quality of the image recorded by the imaging system. Similar types of source follower readout circuits are utilized for reading from other types of addressable nodes, such as locations of memory in memory storage devices. These circuits also have similar noise and gain degradation problems.
In view of these problems, there is a need in the art for readout circuits for addressable nodes that overcome one or more of the technical problems as stated above.